Plested configuration vent structure

ABSTRACT

A vent structure for reliable long term operation at high temperatures is fabricated by annealing a tubing of predetermined length and size, uniformly filling a normally upper portion of the tubing with a predetermined thickness layer of fine alumina particles, flattening and doubly folding and pressing the upper portion of the tubing at predetermined pressures, annealing the formed tubing, and installing a particulate filter in the undeformed normally lower portion of the tubing. For use in venting helium generating reactor control pins located under hot molten sodium, a vent assembly including the vent structure and providing an air lock between the outer molten sodium and the vent is affixed to the upper end of each control pin.

This is a division of application Ser. No. 394,054 filed Sept. 4, 1973,and now U.S. Pat. No. 3,838,557.

BACKGROUND OF THE INVENTION

My present invention pertains generally to venting devices. Moreparticularly, the invention relates to a very effective and practicalvent structure for use in the control pins of a liquid metal fastbreeder reactor (LMFBR), for example, and to a novel method offabricating the vent structure.

As is well known, helium (He) is generated in the LMFBR control pins asthe result of a neutron, alpha (n,α) reaction on boron (B). This heliumis generated in each pin up to the rate of 3 cc/hr or 72 cc/day volumeat standard temperature and pressure (STP). If all of this generated gasis released from the boron carbide (B₆ C) matrix of a control pin, theresulting 26 liters per year poses a serious containment and pressureproblem using closed control pins, especially since sudden onsets ofextreme pressure could develop during the reactor power cycles.Conversely, the use of open control pins allows the boron carbide tocome into direct contact with the high temperature sodium (Na) liquidmetal coolant with the resulting attack thereon and likelihood of boroncontamination of the coolant.

A suitable vent structure for use with a reactor control pin and a ventstructure fabrication method are shown, described and claimed by theinventor in his copending U.S. patent application Ser. No. 296,680 filedon Oct. 11, 1972 for Vent Assembly and now U.S. Pat. No. 3,876,403.While such disclosed vent structure and fabrication method are entirelysatisfactory, a vent structure providing more precise flowcharacteristics therethrough, and which structure can be moreconsistently and easily fabricated to produce an accurate predeterminedflow, is desirable and useful in certain applications.

SUMMARY OF THE INVENTION

Briefly, and in general terms, my invention is preferably accomplishedby providing a vent structure, including a ductile and relatively thinwall tubing having an undeformed normally lower portion and a flattenedand doubly folded and pressed normally upper portion, in a molten sodiumcoolant reactor gas generating control pin, for example, to release itsgenerated gas normally at a relatively low flow rate such that thecontrol pin (gas container) is maintained in a slightly pressurizedstate to prevent backflow of molten sodium or vapor and consequentialcontamination thereof. The formed (flattened and folded and pressed)upper portion of the vent structure preferably includes a thin film orlayer of fine alumina particles between the flattened opposing facesinside the tubing, to prevent the vent from sintering together due tograin growth across the faces occurring at high temperatures and longterm operation. The vent structure preferably further includes aparticulate filter in the undeformed lower portion of the vent tubing.

The method of fabricating the vent structure includes the steps, amongothers, of applying a slight chamfer to the inside edge of the end of anormally upper portion of a metal tubing of predetermined length andsize, annealing the tubing at a predetermined temperature for apredetermined period, uniformly filling the upper portion of the tubingwith a predetermined thickness layer of fine alumina particles,flattening an upper portion of the tubing and then doubly folding andpressing such upper portion at predetermined pressures, and againannealing the tubing at a predetermined temperature for a predeterminedperiod. Finally, a porous metal particulate filter can be press-fittedinto the undeformed lower portion of the tubing. The resultant ventstructure is structurally stable and can operate reliably in a hightemperature environment over a long term, and gas flow thereof issubstantially a linear function of the applied gas pressure differentialup to the point where the elastic limit of the metal tubing is reached.

To fill the upper portion of the tubing with the uniform alumina layerprior to flattening, the tubing is placed with chamfered end downconcentrically about a polished control rod of predetermined diameterand length mounted upright on a flat resilient base. An appropriateamount of alumina particles is added at the top of the tubing which canbe suitably held stationary against the surface of the resilient base,and the alumina particles are packed down around the control rod using avibrator held against the side of the tubing. The base and rod are nextturned to horizontal and the central control rod is withdrawn to leave auniformly packed layer of alumina in the tubing. The normally upperportion of the tubing is then flattened, folded and pressed as describedabove.

For use in venting helium generating reactor control pins located undera corrosive and reactive liquid environment such as molten sodium, avent assembly including the vent structure and providing an air lockbetween the outer molten sodium and vent structure is attached to eachpin to ensure that liquid sodium does not directly contact the vent. Thevent assembly includes an adapter plug mounting the lower end of thevent structure, cover structure attached to the plug and forming an airlock chamber housing the part of the vent structure above the plug, anda baffle made of porous metal provided in the chamber space between theupper portion of the vent structure and a number of small bleed holes inthe cover structure. The adapter plug is shaped to be suitably attachedto the upper end of a control pin, and the baffle allows free passage ofgas while preventing the passage of particulate matter and impeding thepassage of molten sodium. The possibility of sodium backflow isvirtually eliminated by use of the air lock and the anti-splash porousmetal baffle.

BRIEF DESCRIPTION OF THE DRAWINGS

My invention will be more fully understood, and other advantages andfeatures thereof will become apparent, from the following description ofan exemplary embodiment and method of the invention. The description isto be taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational view, shown in section, of a vent assemblywhich is to be affixed to the normally upper end of a reactor controlpin;

FIG. 2 is a fragmentary elevational view, shown in section and enlarged,of a vent structure constructed according to this invention; and

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I and 3J illustrate certain mainsteps in a method of fabricating the vent structure.

DESCRIPTION OF THE PRESENT EMBODIMENT AND METHOD

In the following description and accompanying drawings of anillustrative embodiment and method of my invention, some specificdimensions and types of materials are disclosed. It is to be understood,of course, that such dimensions and types of materials are given asexamples only and are not intended to limit the scope of this inventionin any manner.

FIG. 1 is an elevational view, shown in section, of the upper ventassembly 10 of a control pin 12 which is used, for example, in a liquidmetal (sodium) cooled fast breeder nuclear reactor (not shown). Thecontrol pin 12 and its upper vent assembly 10 are normally fullyimmersed deeply in (under approximately 8 feet of) the molten sodiumreactor coolant operating at temperatures of about 900° to 1100°F. Eachcontrol pin 12 contains a series of boron carbide pellets (not shown)and is a source of generated helium. The vent assembly 10 is, of course,hermetically attached to the upper end of the control pin 12 andgenerally includes a vent adapter plug 14, baffle 16, air lock tube 18,air lock cap 20, and vent structure 22.

The vent plug 14, air lock tube 18, and air lock cap 20 are preferablymade of a material similar to that of the control pin 12 structuralmaterial such as Type 316 stainless steel, which is compatible with ahot sodium (liquid and vapor) environment. Baffle 16 is made of a porousmetal which is resistant to attack by hot sodium. A felt or foam metalcan be used and, in the exemplary vent assembly 10, a Type F-315 nickelfelt metal produced by Huyck Metals Corporation was used. This materialis about 20% dense and allows free passage of gas while preventing thepassage of particulate matter and impeding any passage of molten sodium.

The air lock tube 18 has, for example, four vent or bleed holes 24 whichcan be 0.0135 inch in diameter equiangularly spaced 90° about the airlock tube at a predetermined distance above the lower end thereof. Thevent plug 14 has an axially drilled central hole 26 and the baffle 16also has an axially punched central hole 28. The punched baffle 16 ispacked on the vent structure 22 against the upper surface 30 of ventplug 14 which is joined at its lower end 32 to the lower end of the ventstructure by a standing lip electron-beam weld for maximum cleanlinessand minimal disturbance of the surrounding metal.

When the lower end of the air lock tube 18 is welded to flange 34 of thevent plug 14, the vent holes 24 are located slightly above the uppersurface 30 of the vent plug and directly adjacent to the lower sidesurface of the baffle 16. Welding of the air lock cap 20 to the upperend of the air lock tube 18 forms an air lock chamber 36 containingcover gas which provides an "air lock" effect over the vent structure 22such that liquid sodium does not directly or normally contact the uppervent end 38. The possibility of sodium backflow is virtually eliminatedby use of the air lock chamber 36 and the anti-splash porous metalbaffle 16 in the vent assembly 10 of control pin 12.

The air lock chamber 36 is a plenum chamber which is made large enoughto provide a sufficient reservoir of gas that prevents liquid sodiumwhich might enter the lower bleed holes 24 and into the baffle 16, as inthe event of any sudden fluctuation (loss) in gas pressure due to atemporary reactor temperature change (drop), from ever reaching the ventopening in the upper vent end 38 of the vent structure 22. Of course,the sodium is subsequently forced out of the chamber 36 following thetemperature change as the gas pressure therein builds up to equalizewith the environmental (8 feet of liquid sodium) pressure.

The bleed holes 24 are made adequately small so that they will not admitthe surge of liquid sodium into the air lock chamber 36, which surge canoccur when the control pin 12 is first immersed in the sodium. On theother hand, the bleed holes 24 are made adequately large mainly forconvenience of drilling very small holes in stainless steel with thepresently available drills and methods. In any event, the combined sizeof the bleed holes 24 must be about equal or (and are vastly) largerthan the effective size of the vent discharge opening in the upper ventend 38 of the vent structure 22, and allows gas to escape from thechamber 36 at about the rate that it is being released from the ventstructure.

The vent assembly 10 has general overall dimensions of length A,diameter B, and an approximate gas space length C (connected throughvent holes 24 to the exterior). Illustrative values for these dimensionsare A of 1.80 inches, B of 0.435 inch (maximum), and C of 0.95 inch, forexample. This vent assembly 10 is, of course, to be welded to the upperend of the stainless steel tube (control pin 12) having a 0.395 inchinside diameter which accommodates the lower portion of vent plug 14.Other dimensions of the vent assembly 10 can be proportionatelyestimated adequately from the elevational view of FIG. 1 and, whileapproximate, will suffice for most purposes. The vent has a gas flowrate sufficient to maintain pressure in the air lock chamber 36 and incontrol pin 12 to prevent backflow of sodium and any possiblycontaminating outside cover gas, respectively. FIG. 2 is a fragmentaryelevational view, shown somewhat enlarged in section, of the ventstructure 22. The vent structure 22 includes a thin wall tubing 40having an undeformed lower portion 42, and a flattened and folded andpressed upper portion 44. The flattened upper portion 44 is preferablyfolded at least twice, as shown. A particulate filter 46 is provided inthe lower portion 42 of the tubing 40, and a film or layer 48 of fineparticles or powder is provided between the pressed opposing surfacesinside the upper portion 44. The tubing 40 can be made of any materialsufficiently ductile to survive the flattening, folding and pressingoperations without cracking, and there are appropriate materials readilyavailable for operation or use under a very wide range of environmentaland temperature conditions.

In the interests of compatibility and uniformity, the tubing 40 can bemade of the same material as the control pin 12 structural material.Thus, the tubing 40 can be made of Type 316 stainless steel having aninside diameter D and an outside diameter E as indicated in FIG. 2.Likewise, the particulate filter 46 can be made of Type F-315 nickelfelt metal similar to that of baffle 16 (FIG. 1) and having a length F.The cylindrically shaped filter 46 can be installed in the lower portion42 by press-fitting it into the tubing 40 to a position approximately asillustrated. Exemplary dimensions for the inside diameter D, outsidediameter E, and length F are respectively 0.093, 0.125, and 0.250 inch,for example.

The formed upper portion 44 of the exemplary vent structure 22 has alength G and a thickness H as indicated in FIG. 2. The film or layer 48can consist essentially of, for example, compacted 0.3 micron diameteraluminum oxide (Al₂ O₃) particles. This particle size could be increasedto as large as 1 micron or reduced to as small as 0.05 micron forentirely satisfactory operation. A particle size larger than 1 micronmight provide individual support points during forming (flattening andpressing) of the upper portion 44 and which may prevent the attainmentof very low gas flow rates. Particles sizes larger than 1 micron arenevertheless desirably and effectively used satisfactorily in the ventstructure 22. Of course, a particle size smaller than 0.05 micron maynot prevent eventual grain growth across the proximate faces of aflattened tube at high temperatures. Thus, in regular long term and hightemperature operation, it is possible for the formed upper portion 44 ofthe vent structure to sinter together.

Illustrative dimensions for the length G and thickness H arerespectively about 0.350 and 0.092 inch, for example. The thickness ofthe film or layer 48 can be approximately 0.005 inch, when pressed,although the layer thickness in the exemplary vent structure 22 can begenerally between 0.003 and 0.007 inch for satisfactory operation. Exactpressing parameters and anneal conditions are functions of theparticular size and wall thickness of the tubing 40 used and the desiredgas flow rate. It may be noted that the formed vent structure 22, withor without the layer 48 of alumina particles, can be used independentlyas a gas flow metering device in various applications following suitableflow calibration thereof.

A substantially linear relationship exists in the response in heliumflow rate of the vent structure 22 to change in helium pressure, andhelium flow rate drops linearly as the driving pressure decreases. Flowrate is, of course, lower at the higher temperatures since there areless gas molecules in a fixed volume at such temperatures with themaintained pressure differential. With the helium generation rate percontrol pin 12 (FIG. 1) at approximately 3 cc/hr, for example, the ventstructure 22 provides an average helium diffusion rate of about 1 cc/hrat one atmosphere pressure differential after about 60 thermal cycles.This allows a helium pressure of approximately 4 atmospheres inside thecontrol pin 12 to provide a two-fold advantage. First, pressurizedhelium has a higher thermal conductivity to dissipate radiation heatingin the control pin and, second, the pressure minimizes any possibilityof back diffusion of either sodium vapor or cover gas into the controlpin. The positive helium pressure maintained in the control pin 12 bythe vent structure 22 can drop by a factor of about 2 withoutcompromising the sodium seal provided.

FIGS. 3A through 3J illustrate certain main steps in the method offabricating the vent structure 22. In FIG. 3A, a Type 316 stainlesssteel tubing 40 of 0.125 O.D. and 0.016 inch wall thickness is cut to apredetermined length of 10 inches, for example. A slight chamfer 40a asshown in FIG. 3B is applied to the inside edge of the end to be pressedas a precaution because metallographic inspection has shown a tendencyfor the "mouth" of the vent to be indented during pressing. The tubing40 is then vacuum annealed 15 minutes at 1900°F, for example, andultrasonically inspected for any surface or subsurface flaws. Austeniticstainless steel is virtually unaffected by pure hot sodium attemperatures below 1000°F. In the region between 1000° and 1500°F,however, there is evidence of some measurable attack, particularly atthe grain boundaries. Based on this constraint, the vent tubing 40 wallthickness was selected to be 0.016 inch to provide a safety factor of atleast 3 over the deepest sodium penetration observed at 1500°F.

The annealed tubing 40 is next placed over a control rod or shaft Smounted upright in a resilient block y with the chamfered tubing enddown as illustrated in FIG. 3C. The rod S can be a carefully polishedbrass rod 1.5 inches long and 0.065 inch in diameter, and the resilientblock y can be of rubber, for example. The tubing 40 may be manuallyheld concentrically about the rod S against the block y and anappropriate amount of 0.3 micron average diameter alumina (Al₂ O₃)particles is added at the top of tubing 40 (using a glass funnel) untilthe top of the rod S is at least covered. The rod S extends, of course,above the upper surface of block y for a distance equal to at least thelength of the upper tubing portion 44. The alumina particles are thenpacked down around the control rod S using, for example, a hand heldvibrator K positioned with its output shaft p in contact with the sideof the tubing 40 as shown in FIG. 3D. The vibrator K can be, forexample, a commonly available electric scriber which has an output shaftthat reciprocates axially in and out at 7200 cps. The stroke length onsuch a scriber can be adjusted (for different diameter tubings 40) by aknob (not shown) at the top of the tool. An even yet thin layer ofparticulate ceramic can thus be obtained for an effective grain growthbarrier. By varying the diameter of the control rod S, any requiredthickness of ceramic layer can be provided. It should be noted that thecontrol rod S functions such that it is not critical that the tubing 40need be held very concentrically about the control rod.

The assembly shown in FIG. 3D is turned horizontal and the control rod Sis withdrawn to leave a "tube" of alumina within the tubing 40 asdepicted in FIG. 3E. This partially coated tubing 40 is then transferredto, for example, a Carver hydraulic press for subsequent flattening andpressing operations. A predetermined length of the internally coated endportion of the tubing 40 is flattened in the Carver press at apredetermined flattening pressure. The ceramic layered end portion ofthe tubing 40 can be placed on a tool steel block T1 approximately 1inch wide so that the chamfered end just barely overhangs and a secondblock T2 is placed on top as shown in FIG. 3F. The blocks T1 and T2 aresuitably positioned in the Carver press which is operated at apredetermined pressure of, for example, 2000 psi and released to producea flattened area as illustrated in FIG. 3G. The flattened portion oftubing 40 can be next formed by manually bending it (with a pair ofpliers) into a Z configuration as shown in FIG. 3H, and a final pressingoperation is performed thereon in the Carver press at a predeterminedpressing pressure of, for example, 2000 psi. This produces the back andforth pleated configuration illustrated in FIGS. 3I and 3J and whichcreates a flat tortuous path for the escaping gas.

The compacted alumina throughout the vent wall interface eliminates theoperational sintering tendency due to grain growth across the faces ofthe flattened tubing 40 at high temperatures. The formed tubing 40 isthen preferably vacuum annealed 15 minutes at 1900°F again. Aparticulate filter 46 is finally press-fitted into the formed tubing 40as indicated in FIG. 3J to complete the vent structure 22. The formedtubing 40 is preferably annealed particularly where the vent structure22 is used in high temperature (600°C or 1112°F) operation because flowrate doubles in the vent structure going from a stressed to annealedcondition and operating at the high temperature of 600°C, the vent willself-anneal over a period of 3 weeks.

The alumina or aluminum oxide powder is compatible with any vent tubingmaterial and is uniquely suited for operation in the presence of hotsodium vapor. It is an extremely stable oxide and is the only commonlyprocessed oxide resistant to attack by hot sodium and sodium vapor. Asmentioned previously, however, exact pressing parameters (and annealconditions) are a function of the particular size and wall thickness ofthe vent tubing used and the desired gas flow rate. In the flatteningand pressing operations required on the particular vent tubing 40 of0.125 inch O.D. and 0.016 inch wall, gas flow rate at 15 psidifferential pressure is illustratively varied according to flatteningand pressing pressures as indicated below.

    ______________________________________                                        Flattening Pressure                                                                        Pressing Pressure                                                                           Flow Rate                                          (psi)        (psi)         (cc/hr at 15 psi Δ P)                        ______________________________________                                        For 8 microns average alumina particle size:                                  1000          500          30                                                 1000         1000          15                                                 1400         1400          8                                                  2000         2000          4                                                  4000         4000          2.5                                                For 0.3 micron average alumina particle size:                                 1000          500          6                                                  1000         1000          3                                                  2000         2000          1                                                  4000         4000          0.2                                                For 0.1 micron average alumina particle size:                                 1000          500          2.5                                                1000         1000          1.5                                                2000         2000          0.5                                                4000         4000          0.01                                               ______________________________________                                    

By changes in pressing pressures and techniques, the vent structure 22can be fabricated for any required flow down to, for example, 0.0036cc/hr helium at one atmosphere pressure differential. Also, flow throughthe vent structure 22 can increase greatly in case of a sudden increasein helium pressure within the control pin 12. Thus, the vent structure22 effectively acts to relieve pressure transients in the control pin 12to prevent any possible ruptures thereof and then returns by naturalspringback to normal operation if the elastic limit of the formed tubing40 has not been exceeded. The illustrative vent structure 22 has beentested and found good to over 3500 psi, for example. While the doublyfolded, formed tubing 40 does not actually unfold in order to relievehigh increases in pressure, it tends to do so. This was substantiated intesting a vent structure 22 with increasing pressure until it burst at avery high pressure, when the formed tubing 40 did unfold to some extent.

Because of the production of lithium as a result of the (neutron, alpha)reaction on boron carbide and the possibility of boron carbidedisintegration under irradiation, the unpressed section (lower portion42 in FIG. 2) of tubing 40 can include the particulate filter 46 whichallows free passage of gas. To produce the particulate filter 46, nickelfelt metal can be used. Of course, a foamed instead of felt metal can beused and the metal can be stainless steel instead of nickel, forexample, since such materials are resistant to attack by hot sodium andallow free passage of gas. A filter disc or discs of appropriate sizecan be punched out from the nickel felt metal to form the filter 46.

Baffle 16 can be fabricated from stainless steel foam metal. This bafflematerial can, of course, be nickel instead of stainless steel and feltinstead of foam metal. Indeed, it is usually convenient and preferablethat the baffle 16 and filter 46 be made of the same identical material.The foam metal is punched or otherwise shaped to size and configuration.In similar manner, the vent adapter plug 14, air lock tube 18 and airlock cap 20 can be shaped from a Type 316 stainless steel rod. The plug14, tube 18 and cap 20 can be machined to size and shape from thestainless steel rod. It is, of course, apparent that the formed tubing40, filter 46, baffle 16, adapter plug 14, air lock tube 18 and air lockcap 20 can be fabricated in any elected sequence or all concurrently.

The particulate filter 46 is packed into the formed tubing 40 tocomplete vent structure 22 and the baffle 16 is packed onto the formedtubing. The adapter plug 14 is installed on the vent structure 22against the lower surface of the baffle 16 and is joined at its lowerend 32 to the lower end of the vent structure by a standing lipelectron-beam weld. A helium leak check is made of the electron-beamweld, and the air lock cap 20 is tungsten-inert-gas (TIG) orelectron-beam welded to the air lock tube 18 with its vent holes 24(FIG. 1) located at the opposite end thereof away from the cap. The airlock tube 18 is then T1G welded to the adapter plug 14 to complete ventassembly 10. Completed assemblies can be packaged in plastic bags andsealed for delivery.

The vent structure 22 is, for example, produced for operation in ahostile environment including high purity molten sodium at 600°C and aradiation fluence level of 10²⁰ to 10²² nvt (neutrondensity-velocity-time or neutrons/cm²) with an average neutron energy of70.1 Mev (million electron volts). It is, however, evident that the ventstructure 22 can be readily adapted for use to any application where acontrolled gas release in either a friendly or hostile environment isnecessary. The only requirement is choice of metal tubing and ceramiccompatible with the environment and capable of fabrication into thedesired shape.

While an exemplary embodiment and method of this invention have beendescribed above and shown in the accompanying drawings, it is to beunderstood that such embodiment and method are merely illustrative of,and not restrictive on, the broad invention and that I do not desire tobe limited in my invention to the specific constructions or arrangementsor steps shown and described, for various obvious modifications mayoccur to persons having ordinary skill in the art.

I claim:
 1. A vent structure comprising:a tubing of predetermined lengthand size, said tubing including an undeformed normally lower portion anda formed normally upper portion, and said upper tubing portion beingflattened and folded against itself at least twice in a generally Zconfiguration; and a relatively thin layer of small sized particles insaid upper tubing portion separating its opposing proximate faces andpreventing them from sintering together.
 2. A vent structurecomprising:a tubing of predetermined length and size, said tubingincluding an undeformed normally lower portion and a formed normallyupper portion, and said upper tubing portion being flattened and foldedagainst itself at least twice in a generally back and forth pleatedconfiguration; and means provided in said upper tubing portion forseparating its opposing proximate faces to prevent sintering and similarclosures thereof whereby proper venting of fluid flow can be maintainedthrough said vent structure to produce a reliable, long term operation,vent structure.
 3. The invention as defined in claim 2 wherein saidmeans provided in said upper tubing portion includes afluid-transmissive layer of material for separating said opposingproximate faces to prevent sintering and similar closures thereof. 4.The invention as defined in claim 3 wherein the end of said upper tubingportion is chamfered.
 5. The invention as defined in claim 3 whereinsaid fluid-transmissive layer of material comprises a relatively thinlayer of small sized particles in said upper tubing portion separatingsaid opposing proximate faces to prevent sintering and similar closuresthereof.